This invention relates to the manufacture of artificial teeth. More particularly, it relates to the use of dental porcelains which are used on substrates made of silver-containing alloys where discoloration of the porcelain is a problem.
In the manufacture of dental porcelain restorations, metal substrates are fabricated and utilized as frameworks onto which porcelain is applied and fused. The porcelain forms the tooth or teeth and is intended to be functional and also aesthetic, i.e. to blend with adjacent dentition with respect to shape, contour, shade or color, translucency, etc.
When the technology of fusing porcelain to metal for the purpose of fabricating dental restorations was first developed in the 1950's, the only available appropriate dental alloys contained large amounts of gold. As gold became cost prohibitive, alloys were developed replacing the gold content with palladium and silver. These alloys are more economical and mechanically superior, however the silver contained in the alloy causes the porcelain to become discolored during and after the fusing process.
Opaque, body or dentin, and incisal porcelains are provided to the dental technician or ceramist in powdered form. The opaque powder is mixed with a wetting agent, most commonly distilled water, to a creamy paintlike consistency. It is then applied to an alloy substrate and fired in an oven for the purpose of fusing the ceramic material to the alloy. The body and incisal porcelain powders are each mixed with a wetting agent, most commonly distilled water, to a claylike moldable consistency and then applied to the opaqued metal framework and shaped in the form of a tooth or teeth. The porcelain "build-up" is then fused to the opaqued metal framework by firing it in an oven to the appropriate temperatures for the appropriate amount of time. If the metal framework contains silver, the porcelain will develop a yellow-green discoloration as this is the manifestation of the corrosive effect that the silver has on the porcelain.
In order to effectively eliminate this problem of discoloration, the technician must compromise and limit his/her choice of materials by either using an alloy substrate which is silver-free or choosing from a very limited selection of porcelains that are manufactured in such a way as to inhibit discoloration from silver but which may not be totally effective, esthetically acceptable, technically adequate, or economically feasible. Non-precious alloys are silver-free but are difficult to work with as they are hard to grind and shape, and also they are not universally accepted as being biocompatible with human tissue, as these alloys generally contain nickel, cobalt, chrome, and beryllium. Precious alloys which are silver-free contain greater amounts of palladium than alloys which do contain silver, thus becoming much more expensive. These high palladium, silver-free alloys have a tendency to deform during firing, develop a very dark oxide which is difficult to cover with opaque porcelain, and are difficult to solder because of their relatively low melting temperature. From a technical, economical, and biocompatibility standpoint, dental alloys which contain silver are superior to those which do not.